The present application concerns integrated circuits, especially the structures and processes necessary to integrate dual supply voltages onto a single chip.
Background: Drain Profile Engineering
One of the long-standing problems in submicron field effect transistors is hot carrier effects. In an old-fashioned NMOS single-drain-diffusion transistor structure, the potential energy (voltage) of an electron changes dramatically as it crosses the boundary of the N+ drain diffusion. This sharp change in potential energy, over a short distance, defines a high electric field value, which is undesirable because it causes the electrons (and to a much lesser extent, holes) to behave in a different way within the semiconductor lattice. Electrons which have been activated in this way by high electric fields are referred to as xe2x80x9chot electrons.xe2x80x9d Such electrons can, for example, penetrate into or through the gate dielectric, and can cause the gate dielectrics to become charged up over time, until the transistor fails in service.
One technique to avoid hot carrier effects is lightly doped drain extension regions, or xe2x80x9cLDDxe2x80x9d regions. In this structure, a first light and shallow implant is performed before sidewall spacers are formed on the gate structure, with a second heavier implant afterwards to form the source/drain. The first implant provides only a relatively low conductivity in the silicon, but prevents the channel-drain voltage difference from appearing entirely at the drain boundary. By increasing the distance over which this voltage difference occurs, the peak electric field is reduced, and this tends to reduce channel hot carrier (xe2x80x9cCHCxe2x80x9d) effects.
The trade-off as lighter-doped regions are introduced into the process is that transistor performance decreases due to the increased resistance, so the chip designer seeks to find a balance between these two opposing demands. In more recent years, with device sizes shrinking and voltages dropping, the doses needed for LDD regions have become closer to those used for the main source/drain implant, so that these implants are now referred to as xe2x80x9cMDDxe2x80x9d (medium-doped drain) regions.
Background: Dual-Voltage Chip Architectures
As integrated circuit geometries have shrunk below one micron, the possibility of using two supply voltages has appeared increasingly attractive. In this process option, the central part of a chip (the xe2x80x9ccorexe2x80x9d, which performs the actual electrical functionality which is desired from the chip) can be fully optimized for the current state of process engineering for packing density and highest performance, without regard to the voltage standards required for interfacing to the external world. Since many different chips must communicate, the voltage standard required for external interface tend to change relatively slowly. As of 1997, there was no standard signal level below 3.3 volts which is in really widespread usage, whereas the internal voltages of chips in production or announced are very commonly 2.5 volts, 2 volts, or below.
Thus, process optimization for a dual-voltage chip presents some important problems. First, the use of two different voltages demands that different gate oxide thicknesses be used for the core and peripheral transistors. Second, the basic process design should be determined, as far as possible, by optimization of the core devices, since this is the large majority of the area of the chip; the question is then how to modify the basic optimized process to achieve adequate reliability and performance in the peripheral devices. Of course, this must be done while avoiding such problems as CHC effects and punchthrough.
Since each variation between implants received by the core and peripheral transistors requires the use of two separate masks, optimizing both sets of transistors requires 4-5 additional masks, and is not a cost-effective option. However, tests using identical implants for both the low voltage and high voltage devices, while preferable from a fabrication standpoint, do not provide high-voltage transistors which meet the necessary lifetime and performance specifications. The high MDD necessary for the core transistors causes the peripheral transistors to have too high an electrical field, even though the oxide thickness is increased for these transistors. In fact, a 50% reduction in Idrive was required in testing to obtain the desired CFC lifetime specification for this option.
Dual Voltage Process and Structure
The present invention provides a very simple and flexible approach to fabrication and structure of dual-voltage integrated circuits. The inventors have discovered that integration of a dual voltage process can be accomplished by separately optimizing only the gate length and gate oxide thickness (which were already being done) and the drain extension (LDD or MDD) implant, while other implants remain the same for the two transistor types. This integration process requires the use of only 1 additional mask beyond the mask for separate gate oxides.
One method of separately optimizing the drain extension of the HV transistors is to give this implant a higher energy and lower dose than that of the core device. This gives improved hot carrier resistance and reduced substrate current, due to the peak electric field of the device being moved farther from its surface. This specific modification is not required, however. Since the drain extension implant is optimized for both types of NMOS transistor, a different species of dopant can also be used, or other implant conditions changed.
Advantages of the disclosed structures and methods include the following:
reduces substrate current in HV transistors by more than a factor of 2 in experiments;
uses only 1 additional mask over the mask required for different gate oxide thickness;
cheap to implement;
peak electrical field of HV transistor is subsurface;
no loss in performance.
improves CHC life time;
meets CHC lifetime specifications.